Previous investigations have shown that estrogen plays a significant role in the development and maintenance of bone in mammals. The abrupt loss of estrogen at menopause and subsequent bone loss supports this hypothesis. However, the majority of the investigations to date involve the use of a gonadal castrated rodent model in order to induce a reduced hormone state. With the development of the estrogen receptor-alpha knockout mouse, ERKO, we propose to show the direct role of estrogen receptor-alpha in bone development and maintenance. Our specific aims are to clone the promoter of an osteoblastic-specific (i.e., human osteocalcin promoter) upstream of the ER-alpha cDNA and introduce this transgene into the ERKO mouse. This proposal will investigate the basic regulation of the osteoblast specific promoter-driven estrogen receptor- alpha in various osteoblast-like cell lines by transfection and use of chloramphenical acetyltransferase, Northern, and receptor binding assays. This construct will ultimately be introduced into a C57Blk/J6 mouse by using transgenic technology. The creation of an osteoblast- specific ER-alpha expressing mouse can then be used to measure overexpression of the estrogen receptor-alpha in a wild type genetic background, enabling us to understand estrogen's role in bone development and osteoporosis. The ER-alpha osteoblast expression in an ERKO background would begin to allow us to address the role and action of the ER-alpha gene in bone homeostasis in an ER-alpha negative background.